This invention relates to a method and apparatus for making a multi-component glass fiber preform for fabricating optical fibers for use as transmission lines in communication systems. The term "multi-component preform" denotes a preform composed of a plurality of components.
Conventional optical fibers used as transmission lines of optical communication include silica glass fibers, multi-component glass fibers and ionic crystal fibers. It is required that these optical fibers have low loss, are inexpensive to manufacture and can employ wide band signals. Further, these optical fibers are required to enable easy connection between the fibers and to have a high mechanical strength.
Conventional multi-component glass fibers comprise silica SiO.sub.2 and a dopant composed of at least one metal salt selected from the group consisting of alkali metal oxide such as Na.sub.2 O, alkaline earth metal oxide such as MgO, oxide of lead such as PbO and oxide of lanthanum such as La.sub.2 O.sub.3. Such multi-component glass fibers can be easily fabricated from a preform since they have a relatively low melting temperature. Such multi-component glass fibers have another advantage that they have low loss since Rayleigh scattering involved is kept to a low level.
Various methods have been proposed for making a multi-component glass fiber preform. With these methods, starting materials are first subjected to extremely high purification and then are mixed together. Thereafter, the so mixed materials are melted by heat. Thus, these methods have been found not simple. Particularly, where powder materials such as sodium salt, potassium salt, barium salt and lead salt are employed, it is not so easy to mix the starting materials homogeneously. In addition, it is necessary to melt the mixed starting materials, for example, in a crucible for a long period of time. As a result, impurities tend to be introduced into the starting materials during this melting operation. The resultant multi-component glass fiber preform has often failed to provide for optical fibers with low loss.